In the cigarette packing industry, cigarettes are packed in sheets of wrapping material and/or cardboard blanks on automatic machines, on which the sheets of wrapping material and/or cardboard blanks undergo various folding operations. The packing process is performed on successive machines, which perform various packing steps. For example, a first machine forms packets defined by a sheet of foil wrapped about a group of cigarettes, and by a hinged-lid box formed from a blank; and a follow-up machine wraps the packets in a sheet of cellophane. The packets are therefore transferred between adjacent machines along given feed paths. A feed path is normally defined by a group of conveyors, in which a first conveyor operates at a travelling speed related to a first output rate of the upstream machine and a first packet spacing, and a second conveyor operates at a second travelling speed related to a second output rate of a downstream machine and a second spacing. Very often, the first and second travelling speed differ, so that the packets must be accelerated or decelerated, and the spacing changed, when being transferred between the first and second conveyor.
At times, situations occur in which the first and second speed cannot be maintained constant and vary considerably, even to the extent of being reduced to zero, as in the event of a breakdown of the upstream or downstream machine.